If f(x)=ax+bcx+df(x) = \frac{ax+b}{cx+d}f(x)=cx+dax+b and f(f(x))=xf(f(x)) = xf(f(x))=x, which condition on a,b,c,da, b, c, da,b,c,d must be true?
a+d=0a+d = 0a+d=0
a−d=0a-d = 0a−d=0
ad−bc=1ad-bc = 1ad−bc=1
a+d=1a+d = 1a+d=1